1. Field of the Invention
The present invention relates to an exhaust purification system for an internal combustion engine. In more detail, the present invention relates to an exhaust purification system provided with a selective reduction catalyst that purifies NOx in the exhaust under the presence of NH3, a NOx sensor that detects the concentration of NOx, an NH3 sensor that detects the concentration of NH3 on a downstream side of this selective reduction catalyst, and the like.
2. Related Art
As one exhaust purification system that purifies NOx in exhaust, a system has thus far been proposed in which a selective reduction catalyst that selectively reduces NOx in the exhaust by way of a reducing agent such as ammonia (NH3) is provided in an exhaust channel. For example, with an exhaust purification system of urea addition type, urea water, which is a NH3 precursor, is supplied from an upstream side of the selective reduction catalyst, NH3 is generated by thermal decomposition or hydrolysis of this urea water by the heat of the exhaust, and the NOx in the exhaust is selectively reduced by this NH3. In addition to such a system of urea addition type, for example, a system has also been proposed that generates NH3 by heating a compound of NH3 such as ammonia carbide, and directly adds this NH3. A system of urea addition type will be explained hereinafter.
With such a system, in a case of the effective NH3 amount of the selective reduction catalyst being less than an optimum amount, the NOx purification rate declines from the ammonia being consumed in the reduction of NOx being insufficient, and in a case of being larger than this optimum amount, the ammonia that has become surplus in the reduction of NOx is discharged. As a result, appropriately controlling the injection amount of urea water has been important in an exhaust purification system provided with a selective reduction catalyst. Therefore, in Patent Document 1 and Patent Document 2, systems are exemplified that estimate a NOx purification rate of a selective reduction catalyst, and control an injection amount of the urea water based on this estimation.
With the exhaust purification system of Patent Document 1, the NOx amount on a downstream side of the selective reduction catalyst is detected by a NOx sensor, and the composition of the exhaust flowing into the selective reduction catalyst, more specifically, the ratio of NO to NO2, is estimated from the output value of this NOx sensor and the operating state of the internal combustion engine. Furthermore, based on this composition of the exhaust, the NOx purification rate of the selective reduction catalyst is estimated and the injection amount of urea water is controlled.
In addition, with the exhaust purification system of Patent Document 2, the temperature of the catalyst is detected as an amount relating to the NOx purification rate of the selective reduction catalyst, and the injection amount of urea water is controlled based on this temperature.
However, the NOx purification rate of the selective reduction catalyst varies not only by the aforementioned such composition of the exhaust and temperature of the selective reduction catalyst, but also the degradation state of the selective reduction catalyst. In addition, there is variability in purification performance between individual units. Therefore, it is difficult to always optimally control the injection amount of urea water with exhaust purification systems such as those exemplified in Patent Documents 1 and 2.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2004-100700    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2006-274986